1. Field of the Invention
The present invention relates to an image display medium at which repeated rewriting is possible, to particles for a display device used in the image display medium, and to an image forming device.
2. Description of the Related Art
Conventionally, display techniques such as twisting ball display (display by the rotation of particles which are painted two colors), electrophoresis, magnetic migration, thermally rewritable media, liquid crystal having a good memory property, and the like, have been proposed as image display media at which repeated rewriting is possible. These display techniques have an excellent memory property with respect to images, but have the problems that the display surface cannot display white color such as in the case of paper, and the contrast is low.
The following display technique has been proposed (“Japan Hardcopy” '99, Ronbunshu, pp. 249-252) as a display technique using a toner and overcoming the above-described problems: a conductive colored toner and white particles are filled between opposing electrode substrates, and charges are injected into the conductive colored toner via a charge transporting layer provided at the inner surface of the electrode of the non-display substrate. The conductive colored toner, in which the charges have been injected, is moved by the electric field between the electrode substrates, toward the display substrate which is positioned so as to oppose the non-display substrate. The conductive colored toner adheres to the inner side of the substrate at the display side, and an image is displayed by the contrast between the conductive colored toner and the white particles. In this display technique, the entire image display medium is structured by solid bodies, and the display technique is superior in that the display of white and black (color) can be switched by 100% in theory. However, in this technique, there exists conductive colored toner which does not contact the charge transporting layer provided at the inner surface of the electrode of the non-display substrate, and conductive colored toner which is isolated from the other conductive colored toner. Because charges are not injected into these conductive colored toner, these conductive colored toner randomly exist within the substrate without being moved by the electric field. Thus, there is the problem that the density contrast deteriorates.
In order to overcome such a problem, Japanese Patent Application Laid-Open (JP-A) No. 2001-312225 discloses an image display medium comprising a pair of substrates, and plural types of particle groups which have respectively different colors and different charging characteristics and which are filled between the substrates so as to be movable between the substrates due to an applied electric field. In accordance with this proposed technique, a high degree of whiteness and density contrast can be obtained. The particles proposed therein are structured such that the applied voltage needed to display a black-and-white image is several hundred volts, and by reducing the voltage, an increase in the degrees of freedom in designing the driving circuit is made possible.
However, decreasing the applied voltage used in driving results in the problem that the attraction between the substrates and the particles is decreased, and due to shocks from the exterior and static states over long periods of time, the particles fall off from the substrate. In particular, when particles of a large specific gravity containing a color material whose mass is large are used, the falling off from the substrate becomes even worse due to the specific gravity and the cohesion between particles, and it is difficult to hold a stable display image.